The invention relates generally to the field of soldering methods. In particular, this invention relates to a method for soldering flexible circuit boards with a laser.
In order to increase the reliability, reduce the number of cable harnesses, and reduce manufacturing costs, many car manufacturers are replacing round wires with flat-wire plastic flex circuits. These plastic flex circuits are normally constructed from flexible plastic with flat wires embedded in or laminated to the surface.
Plastic flex circuits provide many advantages over traditional round wire harness connections. They are less complex than their round wire counterparts, they have better thermal dissipation because of their larger surface area, and they are cheaper and easier to construct. Flex circuits also have the advantage of being able to act as signal and power carrying means as well as actually becoming part of the circuits themselves.
In practice, different flex circuits and busses may have to be soldered together so that one main flex bus may have several branch flexes and takeouts. This allows the designer to manipulate the placement and shape of each individual flex circuit. To attain these connections between different flex circuits, the preferred method is to solder them together using a conventional reflow solder method. In this method, a layer of solder paste comprised of solder balls in a petroleum jelly-like flux is sandwiched between the copper conductors of two separate flex circuit pieces. The flex pieces are then fed into a reflow oven that heats the pieces, melting the solder paste, which then hardens and binds the copper conductors together. However, since the reflow peak temperature is usually as high as 210xc2x0 C., this method only works well if the flex is made of high glass-transition temperature (Tg) polymers such as polyamide or nylon. If lower Tg flexes are utilized, they will be severely deformed by the high temperatures of the reflow oven, and will lose their functionality.
Unfortunately, high Tg flex material is much more expensive than low Tg flex material. A low Tg flex material such as PET, which has a Tg of only 80xc2x0 C., can perform the desired functions of high Tg flex material at a much lower cost. These PET flex materials can resist the thermal conditions where they are normally housed, such as the instrument panel of an automobile, but they cannot survive the soldering process. It is desirable to solder low Tg flex circuits without deforming them so that the cost of these circuits can be reduced.
Laser soldering has been tried in the past in soldering processes. One method utilizes a stationary laser with 18-inch optics to solder many copper traces together simultaneously. However, because of the size of this laser, the power is much lower than a small, focused laser. In order to reflow solder paste in a typical application, a laser of this size and of reasonable power may require up to 30 seconds or more of application time. This is extremely inefficient and can result in damage to the plastic flex circuits since the copper traces remain heated for an extended period of time.
In one embodiment of the present method, first and second flex circuits composed of polymer flex substrate are provided. Each flex circuit has a top and a bottom side and at least one contact trace embedded in its surface. An area of solder is provided on the contact trace of at least one of the flex circuits and the flex circuits are positioned so that the contact traces of each flex circuit are substantially aligned. A laser beam is positioned to heat the contact trace to melt the solder and fuse the contacts.
In another embodiment of the present method, first and second flex circuits formed from polymer flex substrate and having top and bottom sides are provided. At least one contact trace is embedded in each flex circuit and a layer of solder is plated onto the contact traces. The flex circuits are positioned so that the contact traces are in substantial alignment and a laser beam is positioned at a point relative to the flex circuits. The laser beam is then moved from the point across the flex circuits over the contact traces to reflow the solder and fuse the traces.
In yet another embodiment of the present method, plastic substrates with top and bottom sides are provided and contact traces are laminated to each substrate. The substrates are positioned such that the contact traces form an alignment area and a layer of solder is applied to at least one of the contact traces. A laser is positioned at a point relative to the substrates and the laser is moved across the substrates so that the beam heats a portion of the contact trace to reflow the solder and fuse the traces.